Because of internal thermal dissipation, the temperature of an electronic ballast for arc discharge lamps increases when the ballast is turned on and decreases, that is, returns to ambient temperature, when it is turned off. Repetition of this operation in meeting daily lighting requirements is referred to as temperature cycling. Even when not being used the ballast is subject to the rise and fall of the ambient temperature. This can be critical to the reliability of operation in an outdoor environment, especially in colder climates, because of thermomechanically-induced forces impressed upon circuit board solder joints.
Solder joint cracking is often associated with electronic components that are not strain-relieved on ballast circuit boards that are encapsulated in, for example, asphalt-based potting compounds. This occurs because of thermomechanically-induced forces on the circuit board solder joints. This strain is the consequence of two things: temperature cycling of the ballast and the substantial inherent mismatch between the coefficients of expansion of the potting material and the electronic components.
Over the temperature range of expected operation, the thermal expansion coefficient of asphalt is nominally 8 to 10 times greater than that of the copper or solder. When the temperature increases, the encapsulating potting material expands, pushing components away from the circuit board; when the temperature decreases, the potting material shrinks, pulling components toward the board. Ultimately, this thermally induced pushing and pulling fatigues the solder joints, causing mechanical failure and subsequent electrical disconnection within the circuit.
One of the major components exhibiting these types of problems are bobbins about which wire windings are wrapped, such as transformers and inductors used in the afore-mentioned electronic ballasts. Existing bobbins have lead-ins that serve as rigid connections between the windings and the circuit board. Generally, these lead-ins are either straight or L-shaped.
It would be an advance in the art if simple modifications could be made to these structures to reduce or eliminate the thermally-induced strain problems caused by thermal cycling.